Besides surgery and chemotherapy, radiation therapy is one of the three main options for treating tumor patients. Over the last years, advances in research and technology led to significant improvements in all fields of radiotherapy. While the majority of irradiation is done by high energy photons, another promising approach is the treatment with ion beams. Due to the different depth dose characteristic of charged particles compared to X-rays, superior dosed distributions in the patient and, therefore, higher tumor control and less side effects can be anticipated for treatments with ion beams.
The treatment with proton and heavy ion beams enjoys a rising interest. The most sophisticated technique in proton therapy is intensity modulated proton therapy, which involves narrow beam spots that are delivered to the patient in a scanning pattern. The intensity of the beam spots is modulated individually, and their relative weights are determined by an optimization algorithm to obtain the best possible treatment plan. This process is called inverse treatment planning, since it solves the problem of automatically finding the best set of treatment parameters for a given (prescribed) dose distribution rather than the other way round, which was the conventional approach in treatment planning systems.
Today inverse planning for protons is based on fast and reliable algorithms for dose calculations (care for Nill, Bortfeld, Oelfke, “Inverse Planning of Intensity Modulated Proton Therapy”, Zeitschrift für medizinische Physik 14(1) (2004) 35-40). However, the physical dose is apparently not the only parameter one should look at in treatment planning for protons and ions, as there is experimental evidence that the biological effect caused by charged particle beams does not depend on the physical dose alone, but also on the energy spectrum of the beam. In other words: the same physical dose delivered by protons or by ions with an atomic number ≧2 of different energy does not lead to the same biological results (e.g. in terms of cell survival). Therefore, it is not sufficient to consider the physical dose alone. Instead, 3-dimensional variations of the relative biological effectiveness (RBE) have to be taken into account (care for Wilkens, Oelfke: “A phenomenological model for the relative biological effectiveness in therapeutic proton beams”, Phys. Med. Biol. 49 (2004) 2811-2825). The RBE is defined as the ratio of the dose of a reference radiation and the respective charged particle dose required to yield the same biological effect (e.g. cell survival). This means, that in inverse planning for intensity modulated radiotherapy with ion beams using scanning beam delivery techniques, the biological effect (RBE*dose) has to be optimized rather than the physical dose distribution.
Some inverse planning methods known in the state of the art take the biological effect into account. However, these methods do not permit a simultaneous multifield optimization. Furthermore, they require long computing times for the iterative optimization of the treatment plan. Additionally, these prior art methods cannot account for the common case of mutually conflicting optimization goals (e.g. high and homogeneous effect in the target and sparing of organs at risk or healthy tissue).